FIG. 1 is a p-type input differential Continuous-Time Linear Equalizer (CTLE) 100 which comprises of input p-type transistors MP1 and MP2 with respective source terminals Vs1 and Vs2, a variable RC (resistor-capacitor) network 101 coupled between Vs1 and Vs2, and current sources 104a and 104b, where the input p-type transistors MP1 and MP2 are coupled to termination resistors 105a and 105b respectively. Current sources 104a and 104b are coupled to power supply node Vcc. Each current sources 104a and 104b may each provide current i/2. Termination resistors 105a and 105b are also coupled to ground (GND). Input p-type transistors MP1 and MP2 receive input signals “in” and “in#” at their respective gate terminals, where “in#” is inverse of “in.” Output signals “out#” and “out” of CTLE 100 are coupled to source/drain terminals of MP1 and MP2 respectively, where “out#” is inverse of “out.”
To adjust pole and zero locations of the CTLE, RC network 101 comprises variable resistor 102 and variable capacitor 103. As bandwidth requirements for the CTLE increase, having a linear resistor 102 (same as variable resistor 102) becomes a challenge. For example, a resistor bank having switchable resistances coupled together in parallel may be linear, but suffer from bandwidth degradation because of large parasitic capacitances on terminals Vs1 and Vs2 caused by the bank of parallel switchable resistances. High sheet resistance also results in lower bandwidth in such a configuration of resistors.
A simple Metal-Oxide Semiconductor (MOS) transistor based programmable resistor, for linear resistor 102, may provide high bandwidth, but it is a voltage dependent resistance suffers from common-mode variation. One configuration of a MOS transistor based programmable resistor is a PMOS with its source and drain terminals coupled to Vs1 and Vs2 and its gate terminal driven by a digital to analog (DAC) converter. However, changing voltages on terminals Vs1 and Vs2 causes the PMOS (or any MOS) device to have changing impedance. Additionally, body effect may make a MOS transistor (and hence MOS resistor) operate in out of linear operation region.